CVE-2025-68785

In the Linux kernel, the following vulnerability has been resolved:

net: openvswitch: fix middle attribute validation in push_nsh() action

The push_nsh() action structure looks like this:

OVS_ACTION_ATTR_PUSH_NSH(OVS_KEY_ATTR_NSH(OVS_NSH_KEY_ATTR_BASE,…))

The outermost OVS_ACTION_ATTR_PUSH_NSH attribute is OKed by the nla_for_each_nested() inside __ovs_nla_copy_actions(). The innermost OVS_NSH_KEY_ATTR_BASE/MD1/MD2 are OKed by the nla_for_each_nested() inside nsh_key_put_from_nlattr(). But nothing checks if the attribute in the middle is OK. We dont even check that this attribute is the OVS_KEY_ATTR_NSH. We just do a double unwrap with a pair of nla_data() calls - first time directly while calling validate_push_nsh() and the second time as part of the nla_for_each_nested() macro, which isnt safe, potentially causing invalid memory access if the size of this attribute is incorrect. The failure may not be noticed during validation due to larger netlink buffer, but cause trouble later during action execution where the buffer is allocated exactly to the size:

BUG: KASAN: slab-out-of-bounds in nsh_hdr_from_nlattr+0x1dd/0x6a0 [openvswitch] Read of size 184 at addr ffff88816459a634 by task a.out/22624

CPU: 8 UID: 0 PID: 22624 6.18.0-rc7+ #115 PREEMPT(voluntary) Call Trace: dump_stack_lvl+0x51/0x70 print_address_description.constprop.0+0x2c/0x390 kasan_report+0xdd/0x110 kasan_check_range+0x35/0x1b0 __asan_memcpy+0x20/0x60 nsh_hdr_from_nlattr+0x1dd/0x6a0 [openvswitch] push_nsh+0x82/0x120 [openvswitch] do_execute_actions+0x1405/0x2840 [openvswitch] ovs_execute_actions+0xd5/0x3b0 [openvswitch] ovs_packet_cmd_execute+0x949/0xdb0 [openvswitch] genl_family_rcv_msg_doit+0x1d6/0x2b0 genl_family_rcv_msg+0x336/0x580 genl_rcv_msg+0x9f/0x130 netlink_rcv_skb+0x11f/0x370 genl_rcv+0x24/0x40 netlink_unicast+0x73e/0xaa0 netlink_sendmsg+0x744/0xbf0 __sys_sendto+0x3d6/0x450 do_syscall_64+0x79/0x2c0 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Lets add some checks that the attribute is properly sized and its the only one attribute inside the action. Technically, there is no real reason for OVS_KEY_ATTR_NSH to be there, as we know that were pushing an NSH header already, it just creates extra nesting, but thats how uAPI works today. So, keeping as it is.

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/10ffc558246f2c75619aedda0921906095e46702
• https://git.kernel.org/stable/c/1b569db9c2f28b599e40050524aae5f7332bc294
• https://git.kernel.org/stable/c/2ecfc4433acdb149eafd7fb22d7fd4adf90b25e9
• https://git.kernel.org/stable/c/3bc2efff20a38b2c7ca18317649715df0dd62ced
• https://git.kernel.org/stable/c/5ace7ef87f059d68b5f50837ef3e8a1a4870c36e
• https://git.kernel.org/stable/c/c999153bfb2d1d9b295b7010d920f2a7c6d7595f
• https://git.kernel.org/stable/c/d0c135b8bbbcf92836068fd395bebeb7ae6c7bef